1. Field of the Invention
The present invention relates to an apparatus for separating data representing a pixel into a plurality of color component data and then performing color image processing in accordance with color component data thus obtained.
2. Description of the Prior Art
In a conventional apparatus for obtaining a color copy image in full color such as a color printer, a color copying machine or a color printing machine, a color image is obtained by superposing yellow, magenta and cyan developers or inks on the same image region. When such a color copy image is formed, the black component of the original image, and the reproduced black component of the copy image, largely determine the sharpness of the resultant image.
In a conventional color copying machine, for example, a black region is obtained by transferring thereto the respective yellow, magenta and cyan developers at a high density and with uniformity. However, the yellow, magenta and cyan developers do not purely consist of yellow, magenta and cyan components alone and have reflection factors which are a function of wavelength or spectral reflectances as shown in FIG. 1. For example, a magenta developer is known to contain considerable amounts of yellow and cyan components, and a cyan developer is known to contain small amounts of yellow and magenta components. Accordingly, even if yellow, magenta and cyan developers are uniformly transferred to the black region, the color balance between the three colors may not be complete and the resultant black region may have a slight hue. Furthermore, since developers of the three colors are superposed, the reflected light from the black region is not pure, resulting in an impure black appearance.
In view of this problem, in a printing machine, a method is adopted wherein a film for reducing undesirable yellow, magenta and cyan components corresponding to a black region is prepared and the respective color components of the black region are attenuated by superposition of the film. Then, the developers are printed on the black region to attain a true black appearance. However, this method can only be used in a big system such as a printing machine and requires advanced techniques. Therefore, this method is not generally adopted for this reason.
In a full color digital color printer wherein respective pixels of yellow, magenta and cyan can be controlled in units of dots, pixels of yellow, magenta and cyan are printed at high and uniform densities in a black region. However, in this case, non-uniform mixing or impurity of the colors can result.
In order to solve these problems, a method has been proposed wherein a black color material is used as the developer and a pixel wherein all of the yellow, magenta and cyan components have levels exceeding predermined threshold levels is determined to be a black pixel. However, since in a black pixel, yellow, magenta and cyan may also be printed in an attempt to reproduce the yellow, magenta and cyan levels, the yellow, magenta, cyan and black components may all be printed on the same pixel. This results in poor printing uniformity or impurity of the black color.
In a full color digital color printer, when a black component is printed by a black developer or ink, the reading system has a configuration as shown in FIG. 2. In this case, exposure light L is divided into respective color components L.sub.Y, L.sub.M, and L.sub.C by dichroic mirrors 5 to 7, which are subjected to photoelectric conversion by CCDs 8, 9 and 10. The electrical signals from the CCDs 8, 9 and 10 are amplified by amplifiers 11, 12 and 13 and are converted into color component digital signals through A/D converters 14, 15 and 16, respectively. The digital signals from the A/D converters 14, 15 and 16 are compared with predetermined digital values stored in dither ROMs 20, 21 and 22 by comparators 17, 18 and 19 which respectively produce final pixel data D.sub.Y, D.sub.M and D.sub.C. It is here assumed that pixel data of logic level "1" corresponds to the printing level. When all the pixel data D.sub.Y, D.sub.M and D.sub.C are at logic level "1", the corresponding pixel is detemined to have pixel data D.sub.BK or be black component. Address generators 23 to 25 provide addresses for reading out the data in respective the dither ROMs 20, 21, and 22 in correspondence with the respective image positions on the image.
When threshold levels T.sub.Y, T.sub.M and T.sub.C, preset in the ROMs 20, 21 and 22 for a pixel containing Y, M and C components in the ratio shown in FIG. 3(a) are exceeded, this pixel is determined to be a black pixel. Since a black region has Y, M and C components of high densities, the overall image becomes dark unless the threshold levels T.sub.Y, T.sub.M and T.sub.C are preset at considerably high levels. When only a black component is considered, the threshold levels T.sub.Y, T.sub.M and T.sub.C are preferably high. However, if the threshold levels of the color components are preset to be considerably high, digital data representing a pixel having the respective color components as shown in FIG. 3(b) does not reach any of the threshold levels. Thus, in spite the fact that an original image may contain a considerable amount of halftone portions, the conversion outputs become "0", resulting in an image with unsatisfactory halftone portions. In view of this problem, the ROMs 20, 21 and 22 store dither patterns to allow reproduction of halftone portions. However, selection of the dither patterns and control thereof involve complex procedures and are difficult.
In a color printer for reproducing a halftone image using dot data of "1" and "0", a halftone image is usually reproduced by the dither method or the like. Since the dither method is well known, a description thereof will be omitted. In such a printer, when the original image is substantially a halftone image, a good reproduction characteristic may be expected. However, if the original image consists of both a halftone image portion and a character or line portion, such character or line will have poor sharpness. For example, if an original is a map in which black characters are printed on a halftone map image, the characters cannot be reproduced sharply. If the characters are to be reproduced sharply, the halftone image portion cannot be reproduced with high precision.